Revolves, pushes air into the case and is up ~ 800U / min is definitely quieter than any hard drive. At ~ 500U / min or less inaudible. In my computer, a fan of this type is installed, plus another with suspension, as well as CPU and GPU fan (i7 3,7GHz, easily overclocked to 3.9 or 4.2 GHz and a GeForce GTX 770). In addition, 5 HDDs are installed, all fans run (even under load) almost in idle mode (ie 500 U / min) and maintain the system so virtually noiseless cooling.
